1. Field of the Invention
The present invention relates generally to buss systems, and particularly to a high speed synchronous I/O interface buss for use in process control systems.
2. Technical Background
Process control systems are employed in a number of applications that include automated manufacturing systems, plant management systems, shop floor control systems, production line control systems, and other process control systems such as paper manufacturing and mail handling systems. Process control systems typically include monitoring and processing equipment remotely located from a central computer. For example, a mail handling systems may include equipment modules such as mail insertion devices, optical readers, mail sorters, mail transport equipment, and fault monitoring equipment that are interconnected, monitored, and controlled using an I/O interface buss. A robust I/O interface buss having multiple nodes for interconnecting individual high speed process control devices is highly desirable.
In one approach that has been considered, a master controller, under the control of a host computer, sends commands over an I/O buss to remotely located slaves using an asynchronous ASCII format. The ASCII format includes a start bit, a stop bit, and a parity bit, in addition to command data. The buss reserves predetermined ASCII control characters for message control. Other ASCII characters are reserved for addressing and commands. In other words, this approach supports overhead functions such as hand shaking, addressing, control messaging, and error checking. One drawback to this approach relates to the excessive bandwidth required to support the overhead functions. As a result, throughput is slowed considerably by error checking and overhead processing latencies. Throughput is slowed even further by the host computer's control of data transfer between the host-memory and the master buss controller. As process control equipment and machinery is designed to run at ever faster speeds, it becomes increasingly important to provide a buss system with suitable speed, band-width, and noise immunity. What is needed is a free-running I/O buss system that is capable of monitoring and controlling remotely located slave devices substantially in real-time.